Score several slits on each of the chicken drumsticks, using a sharp knife. Mix together the lemon zest, olive oil and curry powder, then rub it all over the chicken, working it into the cuts. Season with salt and pepper and place in a shallow dish. Pour over 1½ tablespoons of the lemon juice, cover and leave in the refrigerator to marinate for 2–3 hours.
Preheat the oven to 190°C. Toss the onion pieces with the vegetable oil, then spread it over the base of an ovenproof dish. Place the chicken on top. Roast for 20 minutes, then turn the chicken, spread with the butter and roast for a further 20 minutes.
Place the chicken on a heated serving dish. Pour the rest of the lemon juice into the ovenproof dish and scrape up the onion with a wooden spatula. Pour into a small saucepan, add the stock, heat gently, then stir in the crème fraîche or sour cream and warm through.
Pour the creamy onion sauce over the chicken and serve with lemon wedges.
Serve with fluffy basmati rice – you could toss the cooked rice with the grated zest of a lemon and 2 tablespoons chopped fresh parsley if you like, which will add extra flavour and colour.